Other forms: smocks; smocked; smocking
A smock is a piece of clothing that's worn to protect the clothes it covers, like an artist's smock or an old-fashioned shepherd's smock.
When you wear a smock, you put on a loose, protective garment that resembles an apron with sleeves. When you smock, however, you use a special sewing technique that makes small, decorative pleats or gathers. The Old English root of smock is smoc, "loose-fitting woman's undergarment," which is probably related to the Old English smūgan, "to creep," and the Old Norse smjúga, "to put on or creep into a garment."